During the progression of the inflamed stomach to gastric cancer, parietal cells fail to secrete acid and are re- placed by metaplastic mucous-secreting cells, a process called gastric atrophy. Compelling evidence shows that inflammation, typically caused by Helicobacter pylori (H. pylori), precedes atrophy. Although atrophy itself is a reliable indicator of pre-neoplastic changes in the stomach, the pathogenesis for the development of this pre-cancerous lesion is largely unknown. Parietal cells secrete numerous factors that modulate the growth and differentiation of the gastric epithelium. One favored explanation linking inflammation and progression to atro- phy is due to the loss of these factors. The Sonic Hedgehog (Shh) signaling pathway is one of the main morphogens expressed in stomach although the mechanism by which Shh regulates gastric epithelial cell dif- ferentiation is unclear. Shh is also a known regulator of cytokine and chemokine production, but its role in the development of H. pylori-induced gastritis has not been investigated. Our long-term goal is to understand the pathogenesis of gastric cancer. The objective of this application is to identify the underlying role of inflamma- tion as a trigger for the disruption of epithelial cell differentiation and thus the cascade leading to cancer. The central hypothesis of the current application is that Shh signaling drives gastric epithelial cell differentiation, function and the gastric immune response. Our hypothesis has been formulated on the basis of preliminary and published studies produced in the applicant's laboratory showing that deletion of Shh results in loss of normal glandular differentiation and function, suggesting that Shh is likely to be involved in the maintenance of adult gastric tissue homeostasis. In addition, Shh signaling is required for the development of H. pylori- induced gastritis. The rationale that underlies the research proposed is that, identified Hedgehog signaling tar- gets that are crucial for the development of gastritis and metaplasia may be applied as biomarkers for cancer diagnostics, prognostics and targeted therapeutics. Guided by strong preliminary data, this hypothesis is tested by pursuing two specific aims: 1) How does Hedgehog signaling regulate gastric epithelial cell differentiation in the adult stomach? And, 2) what is the role of Hedgehog signaling in the development of H. pylori-induced gastritis? The hypothesis will be tested using novel mouse models expressing constitutive and inducible pa- rietal cell-specific deletion of Shh and Shh deletion in the whole animal, adoptive splenocyte transfer, H. pylori- infected mouse models, and gastric and T cell cultures. The research proposed is innovative, because it fo- cuses on a novel approach that will allow us to assay changes in gastric epithelial cell differentiation and func- tion in relation to the loss and gain of Shh expression, independent of inflammation. This proposed research is significant because it is expected to provide knowledge required to potentially diagnose and effectively prevent gastric cancer in the pre-neoplastic stage.